The BCL6 gene encodes a zinc-finger transcription factor and is altered by chromosomal rearrangements in its 5' non-coding region in approximately 30 percent of diffuse large-cell lymphoma (DLCL). Recent findings indicate that in approximately 73 percent DLCL. and approximately 50 percent follicular lymphoma the BLC6 gene is also altered by multiple, often biallelic, mutations clustering in its 5' non- coding region. These mutations are of somatic origin and are found in cases displaying either normal or rearranged BCL6 alleles indicating their independence from chromosomal rearrangements. The general goal of this project is to investigate the mechanism of occurrence of these mutations and their role in lymphomagenesis. In particular, the following lines of investigations will be pursued: 1) Mechanism of BCL6 hypermutation. Several observations suggest that BCL6 mutations may be associated with the activity of the Immunoglobulin Variable region (IgV) hypermutation mechanism in B cells. In order to investigate this issue, we will comparatively analyze the frequency of mutation in IgV and BCL6 sequences in: 1) normal germinal-center B cells ii) B cell derived tumors such as Burkitt lymphoma, chronic lymphocytic leukemia, multiple myeloma which display different levels of IgV mutations, 2) Functional consequences of BCL6 hypermutation. Since BCL6 mutations are clustered in the 5' noncoding region of BCL6, it is conceivable that they may have been selected during tumorigenesis for a role in altering BCL6 gene expression. The effect of mutations on BCL6 transcription initiation and progression will be studied in cell lines carrying normal and mutant BCL6 alleles as well as by transfection of wild-type/and mutant/reporter gene constructs in appropriate B cells lines. 3) Role of BCL6 mutation in lymphomagenesis. To test whether BCL6 mutations contribute to lymphomagenesis, transgenic mice carrying NHL-derived mutant BCL6 alleles will be constructed. These mice will be studied for: i) B cell lineage development; ii) tumor development. These studies should provide insights into the mechanism of occurrence and biological role of BCL6 mutation, the most frequent genetic alteration associated with non-Hodgkin lymphoma.